Mobile networks are currently evolving from pure circuit switched networks towards IP-based networks, and can thus seamlessly be integrated into existing IP-based infrastructure, e.g. the Internet, the World Wide Web and other IP-based communication networks.
Within modern 3rd generation mobile networks, besides the Circuit Switched (CS) domain and the Packet Switched (PS) domain, the IP Multimedia Subsystem (IMS) domain has been specified by the 3rd Generation Partnership Project (3GPP) as a flexible service delivery platform architecture for the provision of IP multimedia services. One of the main components of the IMS domain is the Call State Control Function (CSCF), performing session setup and termination control for IMS sessions. Whereas a Proxy-CSCF (P-CSCF) terminates a session path at the border of an IMS domain, the main control functionality is located within the serving CSCF (S-CSCF).
For session control and signalling functionalities, such as session initiation, session modification and session termination for IP-based voice calls (VoIP) or multimedia conferences, the IMS uses the Session Initiation Protocol (SIP). SIP offers services similar to telephony protocols such as the ISUP (ISDN User Part), but in an IP context. An important logical entity within the SIP framework, terminating a SIP session towards a user, is the user agent. User agents initiate session requests and are usually their destination.
After registration of a user agent in an IMS domain, the user agent represents a particular user contact. A user may of course register more than just one contact, each contact typically corresponding to a particular user device via which the user would like to be reachable. Mechanisms for selecting one out of multiple registered contacts make use of contact attributes as there are the user capability and the q-value (priority value), which are provided by the user agents during registration. In case a contact with highest priority can be identified, a session request is forwarded by the S-CSCF to the respective P-CSCF. In case of two equally ranked contacts, the incoming session is forked, i.e. forwarded to both contacts, which generally results in parallel ringing at two user devices.
Within the framework described so far, a particular problem arises due to the fact that nowadays there is often no longer a 1:1 relationship between a user device (a piece of hardware supposed to be located near the user) and a user agent (typically a software module representing call or session terminating functionality). With modern multi-contact user devices, it is now possible for the user to register more than one user agent in the network, so that two or more contacts can be associated with one and the same user device. This is different to, for example, GSM networks, where the terminating point for a call is the user device having the required functionality fixedly built into it.
For efficient and flexible service provision it will in the future be generally desirable to place service control into the IMS domain also for services which so far have been provided by other domains. For example, the call and service control for classical speech services is performed so far by the CS domain. Prospected network configurations intend however to place call and service control for these services into the IMS domain. Then a user might have, for example, an IMS client and a CS client on his device. Now both of them require a user agent, i.e. there are two user agents associated to the same user device. In the case of identical or overlapping user capabilities, the S-CSCF will thus fork an incoming communication request to two or more user agents associated with one and the same user device, resulting possibly in an undefined operational state of the user device. Furthermore, in case of conditions such as ‘user not reachable’ or ‘user busy’, a redirection mechanism might lead to a redirection of an incoming call to another user agent, but in fact to the same device. Similar problems may arise with two or more IMS clients on the same user device, or in case the user has configured its IMS client for reachability from the CS domain, or in still further configurations.
Accordingly it is an object of the present invention to propose an efficient and reliable mechanism that prevents the undesirable effects illustrated above.